At the present time, there are only a few solid waste consuming facilities that produce a valuable product. Most of these are power plants which raise steam by burning the waste and thereby produce electrical energy. In a few other cases, heat is used to thermally decompose the solid waste into gaseous and/or liquid products (pyrolysis). While very few of these are in operation, others are under consideration or development, especially those which burn the incoming waste or a portion of the pyrolysis product to raise heat.
In general, waste pyrolysis processes can be subdivided into two classes. One class comprises those that require pretreatment of the waste feed, such as extensive grinding and/or pelletizing. Another class comprises those requiring little or no pretreatment. The pretreatment step is expensive, so the non-pretreatment class has an immediate economic advantage. Processes using rotary kilns or fluidized beds as pyrolysis reactors can be placed in the non-pretreatment class.
There are two main categories of rotary kilns: direct-fired and indirect-fired. Direct-fired kilns burn fuel inside the kiln. As a result, the waste material feed is exposed to combustion air. A portion of the pyrolysis product is burned, and the remainder becomes diluted with combustion product gas (i.e., flue gas). When pyrolysis products are diluted with flue gas, the downstream treatment units must be large to accommodate a large flow and are prohibitively expensive. Therefore, the direct-fired kiln is not applicable to pyrolysis.
Indirect-fired kilns supply heat to the waste material inside the kiln by exposing the outside of the steel or other suitable metal kiln to combustion. This is done relatively easily by burning fluid fuels (i.e., liquid and/or gaseous fuels) and impinging the flue gas on the outer surface of the kiln. However, fluid fuels are the more valuable of the pyrolysis products, while solid fuel (or char) is the least available. Therefore, solid fuels are the better economic choice for burning to raise pyrolysis heat. In order to use solid fuels, the kiln must be placed inside a solids-burning furnace. Such furnaces are expensive since they require solids handling, as well as combustion and ash collection equipment. The fluid fuel case thus has the advantage of simplicity but the disadvantage of consuming the more valuable fluid fuels. The solid fuel case has the advantage of utilizing solid fuels but the disadvantage of complexity due to solids handling.
There are two main categories of fluidized bed pyrolyzers: the single-bed and the double-bed. Single-bed pyrolyzers, like fluid-fueled kilns, are relatively simple but they cannot burn char. Double-bed pyrolyzers, like solid-burning kilns, can use solid fuels but they are more complex than the single-bed versions. The fluidized bed pyrolyzers have an additional disadvantage when considered for liquids production: the temperature and residence time for a given fluidized bed are fixed within relatively small ranges. However, for a given feed, the maximization of liquid yields requires some control over the time-temperature profile of pyrolysis products. Ideally, one would ask for complete temperature control of the pyrolyzer and rapid quenching of all pyrolysis products. This maximizes liquid product yield by reducing gas yield. The yield of the liquid product is further increased if the light liquid vapors can be passed directly to a condenser and, at the same time, longer residence at high temperatures can be provided for the heavier liquid (tars). It would be very difficult to attain this feature in a fluidized bed pyrolysis unit unless expensive tar collection, separation, and reinjection equipment were added. However, it is possible to do this in a rotary kiln, as will be discussed below.
Based on observations, an ideal reactor for waste pyrolysis to liquid product incorporates the following features:
1. Little or no pretreatment of feed required PA1 2. Capability to burn solid fuels for pyrolysis heat without external solids handling PA1 3. Indirectly fired so that combustion gas does not mix with pyrolysis product flow PA1 4. Control over temperature-time profile of feed PA1 5. Ability easily to return heavy tars to the pyrolysis zone
The inventive pyrolysis reactor, which incorporates the above features, is described below. The reactor, the Double Rotary Kiln Reactor or DRK Reactor, can be used for pyrolysis or gasification of waste materials such plastics or other organic-based solids, or fuels such as coal, wood, oil shale, etc. Production of liquid and gaseous products such as fuels, synthetic crude oil or useful chemicals is possible depending on the settings of certain variable such as reactor temperature, feed rates and feed material(s), and the type of pyrolysis or gasification catalyst used. The DRK has similarities to those disclosed in the various patents, both Canadian and U.S., issued to William Taciuk, for instance U.S. Pat. Nos. 4,180,455; 4,260,879; 4,285,773; and 4,300,961. However, Taciuk's processor is different from the subject invention and does not have the coaxial alignment of feed inlet and combustion gas outlet of the present invention, nor do the Taciuk patents show the removal of pyrolysis gases through the inlet feed, an important feature of the present invention.